The present invention relates to a colour selection mask structure for a colour cathode-ray tube and, more particularly, to a mask structure designed to be held under tension inside such a tube.
The invention is applicable in any type of tube having a colour selection mask and is particularly suitable for tubes whose mask is held under tension by the frame to which it is fastened.
Conventional cathode-ray tubes include a colour selection mask located at a precise distance from the inside of the glass faceplate of the tube, on which faceplate arrays of red, green and blue phosphors are deposited in order to form a screen. An electron gun placed inside the tube, in its rear part, generates three electron beams in the direction of the faceplate. An electromagnetic deflection device, generally placed outside the tube and close to the electron gun, has the function of deflecting the electron beams so as to make them scan the surface of the panel on which the arrays of phosphors are arranged. Under the influence of the three electron beams, each corresponding to a predetermined primary colour, the arrays of phosphors make it possible to reproduce images on the screen, the mask allowing each beam to illuminate only the phosphor of the corresponding colour.
The colour selection mask must be placed in a precise position inside the tube and supported therein during the operation of the tube. The support functions of the mask are achieved by means of a generally very rigid rectangular metal frame to which the mask is conventionally welded. The frame/mask assembly is mounted in the faceplate of the tube using suspension means welded to the frame and interacting with pins inserted into the glass forming the faceplate of the tube.
The tubes, whose faceplates are becoming increasingly planar, correspond to the current trend towards completely flat faceplates. Tubes having such faceplates are produced using a technology which uses a planar mask, supported under tension in at least one direction. Such structures are described, for example, in U.S. Pat. No. 4,827,179, issued to Adler et al., on May 2, 1989.
Inasmuch as the colour selection mask consists of a very thin metal foil, putting it under tension may generate undesirable vibration phenomena in the mask during operation of the tube. Due to the effect of external mechanical vibrations or shock, for example, acoustic vibrations caused by the loudspeakers of the television set into which the tube is inserted, the mask may vibrate at its natural resonant frequency. Consequently, the vibrations of the mask modify the region of impingement by the electron beams on the screen of the tube, the points of impact of each beam then being offset with respect to the associated phosphor array, thus creating a discoloration of the image reproduced on the screen.
U.S. Pat. No. 4,827,179 proposes adding to the surface of the mask means for damping the vibration of the mask. However, the dampers used in that patent have a complicated structure. Likewise, their use is itself complicated, because the means are installed after the mask has been fastened to the frame, thereby complicating the process for manufacturing the tube by adding steps. Moreover, it is not desirable to add elements to the surface of the mask after it has been tensioned, because its small thickness makes it very fragile and fastening elements to its surface may easily damage it.
Therefore, there is a need for a cathode-ray tube comprising, a mask structure with damping means not having the above-mentioned drawbacks.
A tube according to the present invention comprises:
a colour selection mask in the form of an approximately rectangular metal foil, designed to be fastened under tension to a support frame and mounted inside the faceplate of the tube, the mask having a central region with holes and a peripheral region lying between the central region and the edges of the mask, the mask being capable of vibrating independently of the support frame, and
means for damping the vibrations of the mask, these means being placed around the periphery of the mask,
wherein the damping means comprise at least one mechanical oscillator in the form of a metal strip produced by partially cutting the surface of the peripheral region of the mask.